In a Long Term Evolution (LTE) Release 8/9/10 (Rel-8/9/10) system, Physical Downlink Control Channels (PDCCHs) are transmitted in each radio sub-frame, and particularly they are transmitted in first N Orthogonal Frequency Division Multiplexing (OFDM) symbols of a sub-frame, where N may take the values of 1, 2, 3 and 4 with N=4 being only allowed to occur in a system with the system bandwidth of 1.4 MHz, and the first N OFDM symbols are referred here to a legacy PDCCH region, as illustrated in FIG. 1.
In the LTE Rel-8/9/10 system, the control region in which PDCCHs are transmitted consists of logically divided Control Channel Elements (CCEs), where a CCE consists of nine Resource Element Groups (REGs), and the CCE is mapped to the REGs spanning the entire bandwidth using the method based on interleaving of the REGs. An REG consists of four Resource Elements (REs) duplicated in the time domain and adjacent in the frequency domain, where REs for transmitting common Reference Symbol (RS) are not included in the REs of which the REG consists. An REG is defined particularly as illustrated in FIG. 2. Reference can be made to the description in the Technical Specification 36.211 for the particular definition of an REG and mapping of a CCE to REGs.
Downlink Control Information (DCI) is also transmitted per CCE, and a piece of DCI for a User Equipment (UE) can be transmitted in N consecutive CCEs, where N in the LTE system may take the value of 1, 2, 4 or 8, referred to as a CCE aggregation level. The UE performs PDCCH blind detection in the control region to search for a PDCCH transmitted thereto in a PDCCH search space thereof for which blind detection is required, where blind detection refers to that a decoding attempt is made for different DCI formats and CCE aggregation levels using a Radio Network Temporary Identity (RNTI) of the UE, and if there is correct decoding, then DCI for the UE is received. The UE performs blind detection on the control region in each downlink sub-frame in a non-Discontinuous Reception (non-DRX) state to search for a PDCCH.
In a sub-frame of the LTE system, PDCCH search space in which a blind detection needs to be performed for some UE includes two kinds of PDCCH search spaces, which are a Common Search Space (CSS) and a UE-specific Search Space (UESS). The common search space is primarily configured to transmit DCI for scheduling cell-specific control information (e.g., system information, paging information, power-control commands for a group of UEs, etc.), and the UE-specific search space is primarily configured to transmit DCI for respective UE resource assignments. The common search space starts with a CCE serial number of 0, and only two CCE aggregation levels of 4 and 8 are supported in the common search space; and the UE-specific search space starts with a CCE location related to the current sub-frame number, the RNTI of the UE, etc., and CCE aggregation levels of 1, 2, 4 and 8 are supported in the UE-specific search space. Table 1 depicts a search space in which blind detection by a UE is required in a downlink sub-frame, where L represents the index of an aggregation level, Size represents the number of CCEs in which blind detection is required at the corresponding aggregation level, and M(L) represents the number of PDCCH candidates for which blind detection is required at the corresponding aggregation level. FIG. 3 further illustrates a schematic diagram of the blind detection procedure. As depicted in Table 1, a UE needs to attempt on 22 PDCCHs in a downlink sub-frame, including 6 PDCCHs in total in a common search space and 16 PDCCHs in total in a UE-specific search space.
TABLE 1Search space Sk(L)AggregationThe number of PDCCHTypelevel LSize [in CCEs]candidates M(L)UESS16621264828162CSS41648162
Physical Downlink Control Channels (R-PDCCHs) for a relay system are defined in the LTE Rel-10 system to occupy a Physical Downlink Shared Channel (PDSCH), and FIG. 4 illustrates a structural diagram of R-PDCCH and PDSCH resources, where the R-PDCCHs is configured to transmit control signaling from the base station to the relay node, and the PCFICH stands for a Physical Control Format Indicator Channel.
Particularly resources occupied by the R-PDCCHs are configured by higher-layer signaling. Physical Resource Block (PRB) pair resources occupied by the R-PDCCHs may be consecutive or may be inconsecutive. A search space of the R-PDCCHs is defined so that there is no common search space for the R-PDCCHs but there is only a relay-specific R-PDCCH search space, where Downlink grant (DL grant) and Uplink grant (UL grant) are transmitted in a Time Division Multiplexing (TDM) manner:
The DL grant is transmitted in a first timeslot, and the relay node monitors the DCI format 1A, and a DCI format related to a transmission mode, in the first timeslot; and
The UL grant is transmitted in a second timeslot, and the relay node monitors the DCI format 0, and a DCI format related to the transmission mode, in the second timeslot.
Moreover there are two transmission modes defined for transmission of the R-PDCCHs, which are an interleaving mode and a non-interleaving mode respectively, and different search spaces are also defined in the Rel-10 for the R-PDCCHs in the two transmission modes:
The definition of the PDCCHs, the aggregation levels, and a CCE being a unit, in the LTE Rel-8/9/10 system are still applicable in the interleaving mode, where each CCE is composted of nine REGs, the CCE is mapped to the REGs still through interleaving as defined for the PDCCHs, and a search space thereof is designed the same as that of the PDCCHs in the LTE Rel-8/9/10 to start at a location related to a relay RNTI and a sub-frame number; and
The unit of an aggregation level in the non-interleaving mode is a Physical Resource Block (PRB), where there is invariable mapping of a resource occupied by a channel candidate in a search space to an order of PRBs, and the search space thereof is designed so that a starting location of the specific search space always lies in a VRB logically numbered 0, and there are logically consecutive resources occupied by a set of E-PDCCH candidates at an aggregation level.
In the discussion of E-PDCCHs in the LTE Rel-11, it has been determined that there are frequency-domain consecutive (localized) and frequency-domain inconsecutive (distributed) transmission modes for the E-PDCCH, which are applicable to different scenarios. Typically the localized transmission mode is generally applicable to such a scenario that the base station can obtain comparatively precise channel information fed back by the UE and adjacent cell interference will not vary sharply from one sub-frame to another, where the base station selects consecutive frequency resources with a comparatively good quality, according to Channel State Information (CSI) fed back by the UE, to transmit E-PDCCHs for the UE and performs pre-coding/beam-forming to improve the performance of transmission. If no accurate channel information is available or adjacent cell interference varies sharply from one sub-frame to another and is unpredictable, then E-PDCCHs need to be transmitted in the distributed transmission mode, that is, they are transmitted over frequency resources inconsecutive in frequency for a frequency diversity gain.
In summary, there has been absent so far a specific solution to transmission of downlink control information over E-PDCCHs in the localized transmission mode or the distributed transmission mode.